1. Field of the Invention
This invention relates to semiconductor lasers.
2. Discussion of the Related Art
For many laser applications, a narrow band and time-stable output wavelength is required. This requirement is especially important in modern wavelength division multiplexed (WDM) systems in which several adjacent and narrow wavelength channels carry independent communications. In a WDM system, a small drift in the output wavelength of a laser transmitter can cause a communication to be transmitted in the wrong wavelength channel and be effectively lost.
One narrow band laser is a distributed Bragg reflector (DBR) laser. In a DBR laser, a lasing cavity uses a Bragg grating for one or both of the reflectors of the laser""s Fabry-Perot cavity. The Bragg grating functions as a highly wavelength selective reflector. When used as a reflector of a laser cavity, the wavelength selectivity of the Bragg grating causes the laser cavity to lase over much narrower frequency bands than a comparable laser with non-wavelength selective reflectors. Thus, DBR lasers satisfy one of the requirements of WDM systems, i.e., narrow band outputs.
Many semiconductor DBR lasers also incorporate wavelength tunability by adjusting current values passing through the Bragg grating. The current changes the index of refraction of the grating media and thus, the wavelength range that the grating reflects. Thus, by adjusting the tuning current an operator is able to adjust the output wavelength of such a laser, e.g., to cause a laser transmitter to change transmission channel in a DBR network.
Unfortunately, DBR tunable lasers suffer from wavelength instabilities associated with the use of tuning currents to set output wavelengths. One type of instability results from aging of the DBR laser. With use properties of the semiconductor medium of the laser""s Bragg reflector change, and the amount of current needed to select a desired output wavelength changes. This age-induced effect is problematic in WDM transmitters and requires a mechanism for adjusting the tuning current in time to avoid channel jumping as the relation between applied tuning current and output wavelength changes. Another type of instability results from the need to use a current to select the wavelength of a DBR laser. A typical current source has fluctuations in its output current, e.g., because of temperature changes. These current fluctuations cause the reflection wavelength of a Bragg grating being tuned by the current to fluctuate, i.e., cause the output wavelength of a DBR laser controlled by the current source to fluctuate. Both instabilities can cause channel jumping when transmitters for WDM networks use tunable DBR lasers.
Various embodiments of our invention provide narrow band lasers with wavelength-stabilized outputs. The outputs are typically more stable than those of conventional DBR lasers, because the output wavelengths are fixed by the properties of a resonant optical cavity whose resonant wavelength is at most weakly regulated by currents passing therethrough.
In one aspect, the invention features a laser that includes an optical cavity with first and second reflectors and an optical gain medium interposed between the reflectors. The second reflector is a resonant optical reflector that is pumpable to reduce its loss.